Design Innovation

Conceptual Design for Condylar Guiding Features of a Total Knee Replacement

[+] Author and Article Information
Shahram Amiri

Department of Orthopaedic Surgery, University of British Columbia, Vancouver, BC, V6T 1Z4, Canadashahramiri@gmail.com

T. Derek V. Cooke

School of Rehabilitation Therapy, Queen’s University, Kingston, ON, K7L 3N6, Canadaderek@cookes.com

Urs P. Wyss

Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, R3T 2N2, Canadawyss@cc.umanitoba.ca

J. Med. Devices 5(2), 025001 (May 02, 2011) (9 pages) doi:10.1115/1.4003675 History: Received August 04, 2010; Revised February 10, 2011; Published May 02, 2011; Online May 02, 2011

This study investigates the design requirements for guiding features that can be incorporated into the shapes of the femoral condyles and the tibial component geometry of a knee replacement system without occupying the intercondylar space of the joint so that the cruciates can be spared and still produce more physiological motions. A conceptual design for a surface-guided knee is introduced to induce effective guiding both in flexion and extension by novel features incorporated in the shape of the lateral condyle. This design can accommodate preservation of either of the cruciates while deficiencies in the functions of the other are compensated by contributions of the articular geometry in guiding the motion and stabilizing the joint. The preliminary kinematic tests on a prototype demonstrated viability of the features in guiding motion under compression.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

An example of a surface-guided knee design with a simplified shape of the femoral component that consists of a medial ball and a spiral-shape lateral component (a). The matching tibial component was generated by incrementally moving the femur with respect to the tibia from extension (b) to full flexion while removing the intersecting material from the tibial block. The cross section of the femoral condyle (in gray) inside the tibial cavity (in black) for a selected flexion angle (c) and the magnified view of the boundaries between the components (d) show that excessive removal of the material created a gap between the guiding features that affects the engagement between the tibial and femoral surfaces and compromises the effectiveness of guiding features.

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Figure 2

(a) Projection of the lateral tibial contact arc (Tarc) on the sagittal plane was defined by a circular arc that followed the contour of the dome shape of the lateral tibial plateau (Rd=41 mm). This 2D arc along with the center of the medial ball (Sm) defined a virtual conic surface (Tcone). Location of the contact point (P) on Tarc was prescribed by angle α on the sagittal plane. (b) The 3D shape of the contact arc Tarc was defined by intersection of Tcone and sphere Sp. The RP defined passed through the contact point P and perpendicular to curve Tarc at point P. The incremental relative rotation of the femur with respect to the tibia was defined by vector Ip defined on plane RP as the vector sum of the flexion and pivot rotations about the center of the medial ball (Sm).

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Figure 3

The suggested geometric configuration for the guiding surfaces. (a) The lateral contact point (P) continually progresses along the lengths of the contact arcs (Farc and Tarc), which are located over the surfaces of two virtual concentric and coradial spheres (Sp). (b) The rotation axis of the joint (Irot) along with the lateral contact point (P) defines a RP, over which the contact profiles (Ri and Re) are defined as two circular arcs with variable radii. The contact profiles are tangent to the reference line (Lt) that passes through P and is tangent to medial ball (Sm). The common tangent line of the contact arcs (V) should remain perpendicular to RP and Irot so that the kinematic compatibility between the geometries of the surfaces and the target motion can be maintained.

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Figure 4

(a) Radii of the internal (Ri) and external (Re) aspects of the lateral condyle. The difference between the radii of the tibial and femoral contact profiles determines the magnitude of laxities allowed at each flexion angle. (b) The frontal cross section of the joint for a selected flexion angle is shown for the neutral, tibia internally rotated, and tibia externally rotated positions. The internal and external aspects of the tibial cavity (Ri and Re) constrain the rotations of the femoral component within the designed range of laxities.

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Figure 5

The segmented three dimensional model of a sample femur was used to fit the guiding features of the second design to the shape of the lateral femoral condyle. The bearing spacing was converged and the frontal curvature of the frontal radius of the lateral condyle was gradually reduced by increase of flexion angle. These curves on the lateral femoral condyle were used as references to build the final articular surfaces. Details of the dimensions of the geometry are shown in Fig. 6.

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Figure 6

Cross sections of the femoral condyles at different flexion angles for the two designs. In both designs, the medial condyle replicates a ball-and-socket articulation with a constant radius (Rm=24 mm). In design A, variation in the radii of the internal and external aspects of the contact profile (Ri and Re) was used to produce guiding. In design B, the shape of the guiding features on the femoral condyle was defined independent of the target motion and by having variable sizes for the bearing spacing (BD) and the frontal radius (Rl) of the lateral femoral condyle.

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Figure 7

(a) Surface configuration of the prototype tibial and femoral components: (1) the natural patellofemoral articular surface, (2) medial ball, (3) condylar guiding features, (4) anterior stabilizing facet, (5)–(7) smooth transition patches, and (8) posterior chamfer to avoid impingement in high flexion. (b) The medial and frontal views of a sample prototype shown for better illustration of some of the features including the medial ball (2) and the anterior stabilizing facet (4).

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Figure 8

Testing setup for evaluating the kinematics of the prototype: The femoral component of the prototype (1) is attached to the shaft of the AMTI FORCE 5 joint simulator (3) to induce flexion. Parts (4) and (5) are added to the simulator to allow the tibial component to freely move on the horizontal plane. The tibial component (2) is attached to the moving component (5). Component (4) is bolted to component (6) of the joint simulator and moves up and down supported by the hydraulic jack that applied the desired amount of compression. The kinematics of the prototype components ((1) and (2)) were recorded using motion tracking markers ((7) and (8)).

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Figure 9

Contact area calculated for design A, which was built in harmony with the kinematic characteristics of the target motion, and design B in which the shape of the femoral component was defined independently. At the bottom, articulation of the guiding features of design A for the entire range of motion is illustrated.

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Figure 10

Comparison between the target motion (a) and the motion of the prototype under a 520 N compressive force (b). Location of the femur with respect to the tibial component for each flexion angle is illustrated as a line connecting the center of the medial ball to the lateral epicondyle of the femur.

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Figure 11

Diagrammatic views of the guiding features for designs of PCL-retaining (a) and ACL-retaining (b) knees with the condylar guiding features incorporated in their lateral compartments. The posterior views of the knees are shown. The internal and external aspects of the lateral condyles are marked by “i” and “e,” respectively. (a) The external aspect of the lateral condyle engages to compensate the lack of the guiding effects of the ACL during extension by inducing internal rotation and anterior roll forward of the femur. The clearance on the internal aspect of this design leaves the steering during flexion to the PCL. (b) The internal aspect of the lateral condyle actively engages to guide the motion of the joint in flexion, compensating the guiding function of the PCL by producing external rotation and posterior roll back of the femur. The clearance on the external aspect in this option leaves the steering effects during extension to the ACL.




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